The present invention pertains to the stabilization of electric arc furnace dust (EAFD) and, more specifically, the reduction of leaching of heavy metals from EAFD in both acidic and alkaline environments.
The electric arc furnace process is a common steel making practice in use for many years. In a typical electric arc furnace process, solid charge ingredients including raw scrap, lime, burnt lime, iron ore and ferro-alloy additives are placed in a top-charge furnace unit.
A conventional furnace unit is equipped with (1) a roof lift and swing arrangement which permits the roof to swing aside when cold scrap is charged into the furnace, (2) a rocker and rail tilting type arrangement which permits the furnace to tilt forward for tapping and backward for slagging, (3) a system for additions through the furnace roof, and (4) evacuation systems for the removal of dust generated during the steel making cycle.
In an electric arc furnace, electrodes are supported by electrode arms and clamps and project from overhead down through the furnace roof. An electric arc surging between the electrodes and through the furnace charge, typically comprising largely scrap metal, produces heat which melts the charge and refines the steel. The molten steel is tapped, typically at about 3000xc2x0 F., into a ladle and cast into blooms or poured into ingot molds.
In such a process, particulate emissions are generated during (1) the charging of scrap, (2) the tapping of furnaces, (3) the pneumatic injection of additives, (4) oxygen blowing, and (5) meltdown/refining periods. This particulate, which is individually and collectively referred to as electric arc furnace dust (hereinafter EAFD), is typically collected either as a dry waste in baghouses but can also be collected wet, as a sludge.
In its emitted form, EAFD readily leaches heavy metals when wet, producing heavy metals concentrations in a leachate which exceed certain limits as set forth by the United States Environmental Protection Agency (EPA). In fact, EAFD is designated hazardous by the EPA and carries the designation of xe2x80x9cKO61xe2x80x9d as a hazardous material because of the presence of relatively high amounts of leachable heavy metals, such as lead, chromium, cadmium, and thallium. The disposal, transportation, and handling of hazardous materials are more expensive than the disposal, transportation, and handling of non-hazardous materials. EAFD is generally considered one of the more difficult EPA-listed wastes to treat.
The current state of the art for the treatment and disposal of EAFD is either high temperature processing or chemical stabilization/fixation. For technical and economic reasons, the chemical stabilization/fixation treatment is growing rapidly in use and was performed on over one-third of the approximately 875,000 tons of EAFD generated in 1999 in the United States.
There are two sets of standards currently in use by the EPA for land disposal of EAFD; one standard is for placement in secure landfills (land disposal regulations (LDR)), and the second set is for converting the EAFD to a non-hazardous material for placement in a conventional municipal landfill (xe2x80x9cdelistingxe2x80x9d). The current (1998) limits established as LDR are based on the EPA""s SW846 Method 1311, the well-known Toxicity Characteristic Leaching Procedure (TCLP). This protocol involves exposing the material to be tested to an acetic acid solution (at a pH of 2.88) for eighteen (plus or minus two) hours. Recent discussions have indicated that the EPA may be considering additional leaching tests at other pH levels, including higher pH levels.
Delisting protocol for many years required testing by TCLP followed by the Multiple Extraction Procedure (MEP). This procedure is a nine cycle sequence of leaching in a pH 3.00 sulfuric and nitric acid fluid of the solids filtered from the TCLP. This test has been deemed equivalent to exposure to 1,000 years of acid rainfall. For many years, the EPA his discussed the technical weakness of this protocol, emphasizing the unrealistic pH levels used. Very recent petitions for delisting treated hazardous wastes, at the federal level and in U.S. EPA Regions V and VI, have been required to provide testing data for TCLP and MEP tests carried out at leaching fluid pH levels of from 2.88 to as alkaline as pH 12.
In response to each petition filed with the EPA for delisting, the EPA sets limits of heavy metals concentration specific to that petition. The particular limits vary from petition to petition depending on a number of factors. For example, for a delisting petition filed by a company related to the assignee, the EPA granted the petition and set forth the limits shown on Table 1 as generic delisting limits (GDL).
As mentioned above, an EAFD sample undergoing the TCLP must yield a result which is less than the concentration limit specified by the EPA as its Land Disposal Regulations (LDR) in order for the material to be disposed of in a secure landfill. As can be seen in Table 1, the LDRs include thirteen metals. In order to xe2x80x9cdelistxe2x80x9d or reclassify EAFD as non-hazardous according to this granted petition, the sequential testing on a treated waste is the TCLP followed by the nine MEP cycles all on a single sample. Delisting under the above-mentioned granted petition requires that the metals concentrations of fourteen various metals (i.e., the thirteen in the LDR plus vanadium) in a leachate be below specified generic delisting limits (GDLs), as set forth in Table 1. It should be pointed out that there is some benefit or usefulness event for any reduction in heavy metals concentration, even if one or more of the metals concentration exceeds these specific limits.
Both of the tests described above are primarily directed to testing EAFD exposure under acid conditions for the purpose of simulating acid rain conditions. As mentioned above, the EPA has recently begun open discussions directed to the utilization of extraction fluids having a pH above 3.0 for the purpose of heavy metal testing and has recently proposed to grant a delisting petition using other levels, including pH values of approximately neutral and strongly alkaline. The Synthetic Precipitation Leaching Procedure (SPLP) protocol has a fluid pH of 4.2 which is obtained by the addition of sulfuric and nitric acids to water. Moreover, government agencies in Europe and Canada, as well as the American Society for Testing of Materials and the State of California, have developed aqueous leaching tests which are directed to the testing of materials at neutral pH conditions.
Several methods of chemically stabilizing EAFD have been disclosed. For example, U.S. Pat. Nos. 4,840,671 and 4,911,757, entitled PROCESS FOR CHEMICAL STABILIZATION OF HEAVY METAL BEARING DUSTS AND SLUDGES and issued to Lynn et al., disclose methods and mixtures for stabilizing EAFD and similar dusts with fly ash, lime, and water, among other ingredients. These methods partially rely on the pozzolanic characteristics of fly ash to physically entrap the hazardous constituents of EAFD within a cementitiously hardened product.
U.S. Pat. No. 5,245,122, entitled METHOD AND MIXTURE FOR TREATING ELECTRIC ARC FURNACE DUST, discloses a method for chemically stabilizing a hazardous waste composition containing EAFD by utilizing the pozzolanic characteristics of EAFD. This method involves forming a mixture of EAFD with water and lime and, optionally, ferrous sulfate. The freshly blended product has acceptable leachate concentrations when tested by the TCLP protocol. The method disclosed in the ""122 patent minimizes the concentration of certain heavy metals in the leachate from the freshly blended product, even before the mixture cementitiously hardens. U.S. Pat. No. 5,569,152, entitled BUFFERING OF CEMENTITIOUS HAZARDOUS WASTE COMPOSITIONS CONTAINING ELECTRIC ARC FURNACE DUST, specifies the use of dolomitic lime as a pH-controlling agent for a hazardous waste composition including EAFD. The leachate concentrations of heavy metals from EAFD for this method also passed the TCLP protocol.
All of the known methods for trapping heavy metals from EAFD mentioned in the cited patents are primarily, although not necessarily, directed to exposure of EAFD mixtures to acid solutions.
The present invention provides a method and a composition for treating electric arc furnace dust (EAFD). The composition for reducing the solubility of heavy metals from EAFD includes a mixture of EAFD, water, and potassium magnesium sulfate, wherein the mixture further comprises lime from at least one of inherent lime from the EAFD and added lime. In an alternative embodiment, the composition may consist of a mixture of EAFD, water, a magnesium salt selected from the group consisting of magnesium sulfate and magnesium chloride, along with either inherent and/or added lime. According to either embodiment, the composition may also include ferrous sulfate.
The method of the present invention includes forming a mixture of a composition as described above, and then allowing the constituents to react to form a product with acceptable leachate properties. As will be described below, this reaction occurs almost immediately so that heavy metal concentration from a leachate is reduced even in the freshly blended materials, well before the formation of a cementitiously hardened product.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.
In a first embodiment, the present invention utilizes potassium magnesium sulfate as a supplemental agent in a mixture of EAFD and water, along either inherent and/or added lime, to reduce the amount of heavy metals in a leachate resulting from fluid extraction by either acidic, neutral, or alkaline aqueous extraction solutions. According to another embodiment of the invention, a magnesium salt selected from either magnesium chloride or magnesium sulfate is used as the supplemental agent in the chemical stabilization of EAFD.
As discussed above, EAFD is defined as the solid matter emitted from an electric arc furnace. As previously stated, these emissions occur during various phases of the operation of an electric arc furnace including:
(1) the charging of scrap;
(2) the tapping of furnaces;
(3) the pneumatic injection of additives;
(4) oxygen blowing; and
(5) meltdown/refining periods.
EAFD is the dust removed during any one of the above operations or a collection from any combination thereof. EAFD may be collected as a dry waste or wet, as a sludge. In its emitted form, heavy metals readily leach from EAFD when wet, producing heavy metals concentrations in leachates which often exceed the LDR limits as set forth by the EPA.
In determining the concentration of EAFD in the wet mass, it is desirable to have as much EAFD present in the mass without resulting in a leachate which exceeds certain limits, such as the EPA""s LDR, or more preferably GDL, limits as set forth in Table 1. As discussed above, however, these limits, may be revised by the EPA over time and depend upon a number of factors. In addition, the limits are expected to be different for each delisting petition. Thus, the limits set forth in Table 1, although arbitrarily set as a preferred goal in this application, may not be the limits provided by the EPA in subsequently granted petitions. By treating as much EAFD as possible, the total volume of waste is minimized. Therefore, the amount of EAFD added will vary based on a number of factors, including the limits of metals concentration being sought. In an embodiment using a particular EAFD source and using the limits set forth in Table 1 as the desired goal, the concentration of EAFD in the wet mass to be stabilized may be within the range of about 59 to about 82%. (Unless otherwise indicated, all percentages herein will be given as a wet weight basis, i.e., the weight of the constituent divided by the total weight of the mixture, including the weight of any water, but excluding the weight of the supplemental agent, such as potassium magnesium sulfate or a magnesium salt.) Preferably in this scenario, the concentration of EAFD is within about 63 to about 76%.
Preferably, the EAFD used in the present invention has minimal variations between sources generating the EAFD and minimal variations over time within individual sources. Preferably, the EAFD is selected from a single source and/or is well-mixed.
The composition also includes water in an amount sufficient to react with the EAFD. The amount of water could be easily determined by one skilled in the art by balancing the following factors: (1) having enough water for the chemical reactions to occur; (2) permitting some level of compaction to occur; and (3) controlling the dust from escaping the reaction area. For example, an EAFD which has very fine particles might require more water to control the dust. In addition, the upper limit on water addition in the United States is restricted based on a ban, imposed by the EPA, on water bleeding from the treated waste, as measured by a paint filter test. In one embodiment, the sludge only need include about 16 to about 23% water (wet weight basis, i.e. weight of water divided by the total wet weight of the mixture, excluding the weight of the supplemental agent). The preferred water content in one embodiment is about 18 to about 20%. As with the discussion of the amount of EAFD to be added, the amount of water to be added depends on a number of factors, including the limits set by the EPA which may vary over time and in different circumstances. This holds true for the other constituents discussed below.
Lime is also used in the present invention. Any type of lime, as described in U.S. Pat. Nos. 5,245,122 and 5,569,152, incorporated herein by reference, can be used in connection with the present invention. Dolomitic lime is preferred. As used herein, the term xe2x80x9climexe2x80x9d shall refer only to xe2x80x9cavailable limexe2x80x9d as defined in the ""122 patent and shall exclude forms of calcium which are not available for reaction, such as limestone. The term xe2x80x9climexe2x80x9d shall usually mean the total available lime, including the lime already present in the EAFD (xe2x80x9cinherent limexe2x80x9d) and lime which is added for the reaction (xe2x80x9cadded limexe2x80x9d). The amount of lime added to a composition, however, shall be calculated on the same wet weight basis as described above (i.e., weight of lime added (e.g., quicklime, dolomitic monohydrate, etc.) divided by the total wet weight of the mixture, excluding the weight of the supplemental agent.
Some EAFD samples have a sufficient amount of inherent lime therein such that no additional need be added. The primary indicator as to whether any lime must be added is the pH of the leachate from the TCLP. In particular, if this pH is below 9.4, then the mixture should be evaluated for possible lime increase while if it is above 10.2, the mixture should be evaluated for possible lime decrease, such as by neutralization through the addition of ferrous sulfate or some other conventional neutralization technique. If the leachate pH is between 9.4 and 10.2, then no lime need be added in many cases. The desired lime content of the EAFD mixture will vary based on the content of lime inherent in the EAFD, the need for ferrous sulfate, and the supplemental agent used. In EAFD mixtures from most sources, the sufficient lime should be added to achieve a total lime content of between about 2 and 7 wet weight percent in many cases and, preferably 3 to 4%, although these values may vary depending on the conditions, including the desired limits or limits set by the EPA. In general, if lead is leaching at high levels, too much lime was added, while if cadmium or zinc is leaching at too high levels, not enough lime was added. It should also be pointed out that different forms of lime have different molecular weights, and the preferred percentages will vary accordingly. The general ranges provides above are primarily for dolomitic monohydrated lime.
In an optional embodiment of the invention, the lime added to the EAFD mixture is dolomitic lime, which, as defined in the ""152 patent, indicates the presence of magnesium in additional to calcium. When dolomitic lime is selected as the added lime, the amount of dolomitic lime added to the mixture of EAFD will vary with the amount of supplemental agent to be used. More specifically, it should be noted that it is the magnesium in the supplemental agents of the present invention that primarily provide the desired effect. Accordingly, when dolomitic lime is selected as the added lime, a corresponding decrease in the amount of magnesium provided by the added supplemental agent is made to achieve the same effect if the full amount of supplemental agent is added when calcitic lime is selected. As can be appreciated, the reduction in supplemental agent when dolomitic lime is selected will vary but not in a consistent manner because the various supplemental agents of the present invention contain differing amounts of magnesium. In particular, potassium magnesium sulfate contains 8% magnesium, Epsom salts contain 10% magnesium, and magnesium chloride heptahydrate contains 12% magnesium. Dolomitic monohydrate (N) contains about 21% magnesium. One skilled in the art could easily determine the amount of supplemental agent which should be reduced if dolomitic lime is selected. It should be noted that the amount of lime to be added cannot be varied too much, as it is the lime that contains calcium oxide, which is strongly alkaline and helps to attain the desired pH.
Optionally, ferrous sulfate may be added to the mixture. Ferrous sulfate is added to the mixture based on the heavy metal analysis of the EAFD prior to mixing. It is believed that ferrous sulfate serves to reduce hexavalent chromium to trivalent chromium. Trivalent chromium is less soluble than hexavalent chromium and is thus less likely to leach from the cementitiously hardened product. Also, ferrous sulfate assists in the formation of lead sulfate (PbSO4), a less soluble form of lead. Because of this dual role of ferrous sulfate in immobilizing chromium and lead, it is widely used in many compositions. Use of either potassium magnesium sulfate or magnesium sulfate as the supplemental agent will provide the sulfate ion for lead immobilization but not the iron for chromium reduction and subsequent immobilization. While EAFD sources may typically contain insignificant quantities of lead and/or chromium, variability in the quality of scrap metal used in electric furnace steel production makes it advisable to include ferrous sulfate as a safety factor in formulation.
The desired amount of ferrous sulfate is determined easily by one skilled in the art by assessing the effect of ferrous sulfate on chromium and lead concentration of the leachate. Once again, the amount of ferrous sulfate will vary over a wide range based on many factors, including the desired or set limits. In one embodiment, ferrous sulfate may be added in a range of from about 0.5 to about 3.5%. Ferrous sulfate may be added in crystalline form or as a solution thereof. As described previously, the heavy metal content of the EAFD is analyzed prior to mixing; accordingly, no ferrous sulfate would be needed if it was determined that the waste (and the source scrap metal) includes little or no hexavalent chromium or soluble lead or if the amount of lead could be stabilized by selecting potassium magnesium sulfate or magnesium sulfate as the supplemental agent.
Another additive according to a first embodiment of the present invention is potassium magnesium sulfate, which serves as a supplemental agent in both acidic and alkaline environments. As used herein, the term xe2x80x9cpotassium magnesium sulfatexe2x80x9d refers to a compound which includes potassium, magnesium, and sulfate, which may either be hydrated or anhydrous, and which is normally crystalline. Some exemplary potassium magnesium sulfates are langbeinite (K2SO4.2MgSO4), leonite (K2Mg(SO4)2.4H2O), and picromerite (K2Mg(SO4)2.6H2O), with langbeinite being most preferable.
As an alternative to the first embodiment, other magnesium salts have also been found to be effective supplemental agents in a mixture of EAFD, water, and lime. These salts include magnesium sulfate and magnesium chloride. Any known commercially available grades of these magnesium salts may be used, and they may be hydrated or anhydrous. For example, epsom salt, a hydrated form of magnesium sulfate (MgSO4.7H2O), can be used as the magnesium salt.
Yet another optional additive is fly ash. Fly ash is the fine ash produced by the combustion of powdered coal. It does not appear that fly ash plays a significant role in this invention and, if added, may be added in a range of about 1% to about 6% (by weight) in one embodiment.
The composition of the present invention results in reduced heavy metals concentrations of a leachate of the composition following either the EPA""s Toxicity Characteristic Leaching Procedure (TCLP), the Multiple Extraction Procedure (MEP), and the Synthetic Precipitation Leaching Procedure (SPLP), as well as when the aqueous extraction solution is non-acidic, such as at neutral pH or above. This indicates that the invention will perform well in both acid rain and alkaline conditions. In addition, this invention has the potential to retain heavy metals under basic extraction conditions, demonstrating that it will retain heavy metals when the mixture is exposed to alkali soil conditions. In this application, the term xe2x80x9caqueous leaching testxe2x80x9d refers to the EPA defined protocols discussed above, leaching tests performed at neutral pH, leaching tests performed at basic pH, and any other leaching protocol in which mixtures or products are contacted with water for the purpose of measuring the composition of the leachate. xe2x80x9cAqueous extraction solutionxe2x80x9d refers to any solution with a water base and used to extract heavy metals from an EAFD mixture. Because the pH of distilled water can vary in the laboratory setting depending on distillation techniques and water supply, the xe2x80x9cneutral pHxe2x80x9d of water is considered to be within the range of 5.5 to 7.0 for the purpose of this application.
In the past, EAFD mixtures which are buffered to maintain a pH within the range of 9.4 to 10.2 have been found to retain the heavy metals when soaked in aqueous extraction solutions having a pH of about 3.0. Several mixtures of EAFD have been shown to maintain a pH within this range when placed in contact with aqueous extraction solutions of acidic pH, but do not demonstrate such characteristics when placed in contact with acidic solutions with a pH higher than 3.0 or with water. The present invention demonstrates that the use of potassium magnesium sulfate or magnesium salts as supplemental agents in a mixture of EAFD, lime, and water either: (1) maintains the mixtures"" pH within the 9.4 to 10.2 range when the mixture is exposed to both acid and non-acid aqueous solutions; or (2) nonetheless minimizes the leaching of heavy metals even if the mixture is outside of this range.
As mentioned above, the amount of EAFD, water, lime, and ferrous sulfate in the mixture is generally given as a weight percent basis, i.e. weight of constituent divided by total weight of the mixture, excluding the weight of the supplemental agent. The weight percent of supplemental agent is calculated as a percent of the total weight of EAFD, water, added lime, and any ferrous sulfate or other additives but excluding the weight of the supplemental agent itself. For example, a weight percent of 25% of supplemental agent would indicate 25 grams of supplemental agent in 100 grams of EAFD, water, and added lime (with optional ferrous sulfate). This weight percentage shall be characterized herein as an xe2x80x9cincremental weight percentxe2x80x9d. The amount of supplemental agent used varies over a wide range and depends on the particulars of the EAFD being treated, the type of lime used, and the desired extent of reduction in heavy metals concentration, among other factors. In general, the amount of supplemental agent required is that which tends to achieve a pH of a leachate between about 9.4 and 10.2 from any aqueous leaching test, although this is not necessary in all cases. A preferred range of supplemental agent is between 5 and 30% supplemental agent (as an incremental weight percent), and a more preferred range is between 6 and 20%, with a most preferred range between 8 and 15%.
Prior to mixing the EAFD with the other components of the invention, the heavy metal and calcium oxide concentrations of the EAFD are tested. If significant deviations in calcium oxide concentrations are detected in the EAFD, based on a comparison with historical measurements, then the weight percent of lime or EAFD concentrations in the final mixture will be adjusted. In particular according to one approach, with a new sample of EAFD, the amounts of ferrous sulfate and water are fixed based on the amount of chromium and lead levels in the stream. Next, the amount of lime to be added is determined, typically by empirical testing with the goal primarily to achieve the leachate pH to within the desired range of 9.4 to 10.2. As mentioned above, the amount of lime to be added is a function of the available lime inherent in the EAFD and varies among EAFD sources. In some cases, it has been found that the amount of lime to be added is not reduced in a one to one ratio based on the amount of available lime inherent in the EAFD but instead the amount of lime to be added might be reduced in a greater than one to one ratio, such as 1.5:1. Thus, in these cases, with a reduction of one percent of available lime inherent in the EAFD, the amount of dolomitic monohydrated lime to be added is reduced by 1.5%. Part of this is due to the difference in molecular weight of the two types of lime per CaO equivalent. The amount of calcium present in the water used to form the mixture is also routinely tested but has been found to have little or no effect on the ability of the final product to retain heavy metals upon leaching or upon the weight percent of the individual components in the mixture needed to achieve the desired quality of the mixture.
The order of addition of the constituents does not appear to be critical. For example, the constituents (i.e., EAFD, water, lime (if any added), the supplemental agent, and, optionally, ferrous sulfate) may be added together simultaneously or may be premixed in any combination. It is not believed that any intermediates between two or three of the constituents are formed which are critical to the operation of the composition and method of the present invention. In one embodiment, the order of addition is to blend the dry ingredients (i.e., EAFD, lime (if any added), the supplemental agent, and, optionally, ferrous sulfate) well, then add water, to ease the mixing effort. (It should be noted that although lime, the supplemental agent, and ferrous sulfate are referred to as xe2x80x9cdry,xe2x80x9d they may be added in a hydrated form, as contemplated above.)
In addition, the manner of mixing the materials is not important and any conventional means can be used to attain a well-mixed composition. For example, EAFD mixtures are mixed by adding the EAFD, water, lime (if any added), the supplemental agent, and optional ferrous sulfate in a container large enough to accommodate all components. For instance, approximately 40 tons of material can be placed in a large mixing vessel with a back hoe attached. The wet sludge is then mixed for 30 minutes or until the mixture is adequately homogeneous.
The process steps of the present invention, namely forming the mixture and permitting it to undergo the initial chemical reactions which occur almost immediately, can be carried out in a known manner. The only qualification to the first step is that the constituents be well mixed. The step of permitting the mixture to react to form a product having a decreased heavy metal concentration in a leachate is accomplished almost immediately by chemical reactions. Thus, shortly after the mixture is formed, the heavy metals concentration in a leachate is xe2x80x9creducedxe2x80x9d in that the heavy metals concentration in a leachate of the mixture is less than the concentration of a leachate of EAFD if it were simply diluted with inert additives. The xe2x80x9cleachatexe2x80x9d can be that formed by any of the aqueous leaching tests discussed herein, which involve washing the mixture with an acidic or alkaline aqueous leaching solution.
Even immediately after mixing, the mixture of the present invention typically has reduced leachate characteristics, more preferably, below the EPA""s LDRs, after undergoing the EPA""s TCLP, MEP, SPLP, and aqueous leaching tests using alkaline aqueous leaching solutions. Most preferably, the mixture of the present invention has leachate characteristics below the EPA""s GDLs, after undergoing any of these aqueous leaching tests.
After mixing the constituents, the mixture may be permitted to react (or cure) to form a cementitiously hardened product. The reaction may be accomplished, for example, by permitting the mixture to react for seven days at 100xc2x0 F. More commonly, field curing time, equivalent to seven days at 100xc2x0 F., is used. For example, sixty days at 50xc2x0 F. has also been deemed equivalent. A typical field curing time is thirty days at 73xc2x0 F. The experiments reported in the examples below were not permitted to react prior to being subjected to the leaching procedures. Formulations according to the present invention are designed to produce a composition that can be sampled and tested immediately after blending, long before pozzolanic chemistry can contribute to immobilization.
The present invention and the effect of varying the composition of the mixture are more clearly demonstrated by the following examples. The following examples are included to more clearly demonstrate the overall nature of the invention. The examples are exemplary, not restrictive, of the invention.